This invention relates generally to apparatus for data transfer with a rotating data storage disk such as a flexible magnetic disk having a multiplicity of concentric annular record tracks, and more specifically to a system in such a rotating disk data storage apparatus for holding the count of a track counter unaltered despite power suspension.
The rotating disk data storage apparatus, or disk drive according to common parlance, has a transducer or transducers for reading and writing digital data on one or both major surfaces of the disk. Retrieved from the disk by the transducer, the data is directed through an amplifier, a filter and a differentiator to a comparator thereby to be shaped into form suitable for further processing.
Data retrieval from magnetic disks needs special consideration because of the fact that the record tracks on the disk become progressively less in diameter from the radially outmost one inward, that is, that the recording density of digital data becomes progressively higher from the radially outmost track inward. Consequently, were it not for the filter, the differentiator would produce different output waveforms for the same datum depending upon its radial position on the disk. The resolution of the differentiator would then deteriorate as the transducer travels radially inwardly of the disk.
The filter on the preceding stage of the differentiator represents a solution to this problem. The filter constant is varied in order to compensate for the difference in the resolution of the differentiator depending upon the radial position of the transducer on the disk. For instance, in a disk drive for use with a five and a quarter inch disk, the filter constant is set at one value when the transducer is on Tracks 0-43, and at another when the transducer is on Tracks 44-79. Further, in writing data on the disk, the write current for Tracks 44-76 is made approximately 20 percent less in magnitude than that for Tracks 0-43.
The track counter is a standard component of the disk drive, being needed for changing the filter constant and the write current magnitude depending upon the radial position of the transducer on the disk. Despite its name, however, the track counter does not actually count the tracks being traversed by the transducer. As is well known, the transducer travels from track to track on the disk as a bidirectional stepper motor rotates by small increments determined by stepping pulses and in a direction determined by a stepping direction signal. Both stepping pulses and stepping direction signal are supplied from the host system controlling the disk drive. Reset each time the transducer is recalibrated on Track Zero, the track counter counts the stepping pulses in an increasing or a decreasing direction depending upon the stepping direction signal, thereby indirectly counting the tracks being traversed by the transducer. The host system is supplied with this count and thus knows at every instant the current transducer position on the disk.
Usually, in personal computers, word processors and like data processing systems comprising a disk drive, the disk drive has no power switch of its own but is powered on and off with the host system. Recently, however, systems have been developed in which the host system disconnects the disk drive from the power supply when it is not in use, with a view to saving power.
Suppose that the disk drive is powered off while the host system remains on. Conventionally, being comprised of flip flops, the track counter was then incapable of holding the count. The track counter was reset when subsequently powered on. Then the transducer traveled to Track Zero as the host system delivered a recalibration command to the disk drive, so that the count of the track counter agreed with the transducer position on the disk.
The recalibration of the transducer each time the disk drive is reconnected to the power supply is objectionable because of the waste of time and electric energy. Reading or writing of data on the disk will be resumed far more quickly if the transducer traveled to a destination track directly from its position when the disk drive was powered off, instead of after being recalibrated on Track Zero.
Conventionally, however, the count of the track counter disagreed with the transducer position on the disk and with what the host system knew as the transducer position if the transducer was not recalibrated when the disk drive was powered on. It then became impossible for the track counter to correctly change the filter constant and the write current magnitude according to the radial position of the transducer on the disk.
The inconvenience discussed above is not limited to systems wherein the host system causes disconnection of the disk drive from the power supply. In some systems the disk drive may be separably connected to the host system and powered off for reasons other than the inherent function of the host system. When the disk drive is subsequently powered on, the count of the track counter also disagreed with the transducer position on the disk and with what the host system knew as the transducer position if the transducer was not recalibrated.